1. Field of the Invention
This invention relates to the large scale purification of raw natural gases, specifically separating contaminates from amine. Purification plant operators have ordinary skill in the art to which this invention relates.
2. Description of the Related Art
Natural gas, as it is captured from naturally occurring deposits, is composed primarily of methane (CH4) and ethane (C2H6). Certain contaminants are naturally present in the gas, and must be removed prior to delivery of the purified gas for private or commercial use. These contaminants include inorganic carbon dioxide (CO2), hydrogen sulfide (H2S), as well as organic compounds, including light aliphatic hydrocarbons (propanes, butanes, pentanes, and their isomers), aliphatic hydrocarbonsxe2x80x94those containing six or more carbon atoms, and aromatic hydrocarbons.
The aromatic hydrocarbon impurities in raw natural gas consist of benzene, and various forms of alkylbenzenes, including ethylbenzene, toluene, and xylenes. These compounds are referred to collectively as xe2x80x9cBTEX.xe2x80x9d The removal and disposition of these BTEX compounds is particularly problematic (and extremely important) as these compounds are known to be carcinogenic, even in small quantities, and are regulated as such by various governmental agencies. See Ramaratnum, paragraph 1. (Ramaratnum, Munkund; Cartwright, G. Tom, Graham, James R.; xe2x80x9cRecovery of BTEX and other VOCs in the refinery;xe2x80x9d Hydrocarbon Technology International Quarterly; Autumn 1995; p. 96-9. In order to meet these strict environmental standards, an effective method of removing and destroying the contaminants from these impure or xe2x80x9csourxe2x80x9d gases is needed.
Removal and disposal of the BTEX components, as well as the aliphatic compounds with more than six carbon atoms, referred to as volatile organic compounds (xe2x80x9cVOCsxe2x80x9d)xe2x80x94also known have some carcinogenic characterxe2x80x94is customarily accomplished by two processes: vent gas incineration and carbon bed absorption. Both processes begin with stripping the BTEX components and VOCs from the natural gas stream by use of a liquid amine solution. The amine is then carried to a desorption zone. This process is similar to that described in FISCH, (U.S. Pat. No. 4,025,322 May 24, 1977).
In the vent gas incineration process, the BTEX and VOCs are combusted at temperatures in excess of 1200xc2x0 F. An incinerator exposes the vent gas stream to a direct flame that is produced by igniting fuel gas, providing excess air via a forced draft fan. The products of the combustion are carbon dioxide and water vapor, both of which are environmentally acceptable alternatives to BTEX emissions.
Vent gas incineration solves the problem of removing and disposing of BTEX components in the vent gas stream, but can be cost prohibitive from an initial investment and fuel usage standpoint, since this conventional process requires specialized, expensive equipment, and dramatically increases annual fuel usage of the purification plant. Also, vent gas incineration is disruptive to project planning, since it usually requires an approximately six month long permitting process to obtain a federal permit for the incinerator.
A second conventional process involves removing the BTEX and VOC contaminants from the amine stream and passing them through a bed of granular activated carbon (GAC) which adsorbs the harmful organic compounds. This method was described in Ramaratnum. The GAC may then be regenerated with steam to liberate the adsorbed BTEX components, which are then cooled and captured as solid products. Disadvantages to this method include an extremely high installation cost, a high energy cost required to dehumidify the waste gas stream, and problems associated with disposal of the solid hazardous waste residue which is produced in the process. (See Ramaratnum, p. 99).
It has been suggested that the present invention is rendered obvious by the presence, in the prior art, of certain dehydration processes in the petrochemical industries. To address and correct this concern, the discussion of Appendix A is provided.
An object of this invention is to provide an economical and feasible method and associated equipment for purification of raw acid gas streams by removing and destroying harmful aromatic hydrocarbon and heavy aliphatic hydrocarbon contaminants, and lowering levels of H2S and CO2.
A further object is to prevent wholesale contamination to the environment from large accumulations of toxic waste material composed of harmful carcinogenic compounds by providing a safe, effective, reliable, and inexpensive manner in which to safely destroy those toxic compounds into environmentally safe products.
Further objects are to achieve the above objects with a device that is sturdy, compact, durable, lightweight, simple, safe, efficient, versatile, ecologically compatible, energy conserving, and reliable, yet inexpensive and easy to manufacture, install, operate, and maintain.
Other objects are to achieve the above objects with a method which is rapid, versatile, ecologically compatible, energy conserving, efficient, and inexpensive, and does not require highly skilled personnel to install, operate, and maintain.
In satisfaction of these and related objects, this invention provides an improved method and structure for purification of an acid gas stream by using raw fuel gas as a stripper to remove the BTEX and VOCs from the liquid amine stream. The improved method is particularly useful for purification of acid gas streams with BTEX contaminant levels in excess of environmentally acceptable levels for standard processing.
Applicant has discovered that if raw fuel gas is utilized at moderate temperatures and pressures, as discussed below, the uptake of BTEX and VOCs reduces the level of these compounds in the waste amine stream to environmentally acceptable levels, and the remaining contaminants may then be dealt with by ordinary means. The invention is also effective at reducing the levels of H2S and CO2. in the liquid amine stream.
Applicant has also discovered that the BTEX and VOCs liberated by this process may be carried away with the stripping gas and burned in the heater used to provide energy to drive the purification process. This has the advantages of destroying the contaminants in an environmentally acceptable manner, to produce environmentally acceptable alternatives. Burning the stripping gas in the process heater also has the advantage of lowering the cost of the purification process because the stripping gas is recycled as fuel gas to fuel the process.
The improved process is based on the preference of BTEX components for the gas phase as the pressure is lowered in accordance with Henry""s law equation: Y=HX/P where Y is the concentration of the component in the gas phase, H is a standard physical Henry""s law constant, X is the concentration of the component in the liquid phase, and P is the pressure.
The specific nature of the invention, as well as other objects, uses, and advantages thereof, will clearly appear from the following description and from the accompanying drawings, the different views of which are not necessarily scale drawings.